Electrical bandpass filters allow certain frequency components of an input signal to pass unattenuated from the input terminals to the output terminals, while all components having frequencies outside of the passband are attenuated. In environments where accuracy of the passband is required, the bandpass filter may need adjustment in order to meet the specifications of the individual applications and customers. Such adjustments may be required for a number of different parameters. Several of the more critical parameters include: frequency accuracy, return loss, insertion loss, and bandwidth. The need for the physical filter to meet the desired parameters becomes even more critical with narrow band filters of the type utilized in connection with wireless communications applications such as cellular and PCS.
Unfortunately, the need for adjustment of the these narrow band filters can not be easily eliminated by design due to manufacturing limitations and physical characteristics. More specifically, the manufacturing and physical characteristics change from physical device to physical device due to dielectric constant variations of the substrate; thickness variations of the substrate; variations of the photolithographic pattern; and the aging effect due to temperature change and cycling of the filter.
One method which has been previously utilized to adjust RF filters is to utilize moveable conductors or dielectrics, such as tuning screws or spring pins. This approach is described in Socher, U.S. Pat. No. 5,968,875. In this type of system, a screw is used in connection with the capacitor type element. While this arrangement may be used to solve for dielectric constant variations of the substrate, the thickness variations of the substrate, and the photolithography variations of the photolithographic pattern, this type of tuning can not be utilized to solve for the aging effect due to temperature changes and cycling.
It will be appreciated that filters which are utilized at a constant or room temperature do not necessarily have a problem with an aging effect due to the temperature change in cycling. More specifically, filters used in these types of environments can be retuned at room temperature once the aging effect occurs. However, since cryo-cooled filters have been utilized, there has been a need in the art to develop a method and apparatus for tuning the filters dynamically without removing the filter from its cooled environment and while the filter is cooled.